A fundamental problem in developmental neurobiology is how the relationship between cell proliferation and pattern formation is coordinated. Studies during the previous funding period have identified a message form of the D2 cyclin gene, MN20, in selected neural precursors making their final divisions and in recently post-mitotic neurons. In cerebellum, MN20/D2 cyclin is expressed in the ventricular zone and is up-regulated in post mitotic cells, such as those contributing to the deep cerebellar nuclei. While absent in the granule cell precursors as they originate and migrate from the embryonic rhombic lip, MN20/D2 cyclin is heavily expressed in these cells only after they reach the external germinal layer in the early postnatal period. Therefore, D2 cyclin expression in granule cell precursors coincides with the time and location in which these cells become competent to differentiate. Its developmental expression pattern suggest that D2 cyclin plays a role in the final rounds of division and differentiation of selected neurons. Preliminary investigation using antisense oligonucleotides indicate that down-regulation of D2 cyclin in granule cell precursors reduces BrdU incorporation and prevents neurite extension. This project will test the hypothesis that the expression of cell cycle regulatory genes is important for primary neurogenesis and regional neuronal differentiation. Specifically, our goal is to examine how cell division is regulated during cerebellar histogenesis and to establish whether cell cycle components, like D2 cyclin, can interact with developmentally important gene products to influence cellular differentiation and organization. Aim 1a will test the role of cell cycle progression and cytokinesis in neuronal differentiation by disrupting the cycle at various points using antisense oligonucleotides to reduced expression of other regulatory proteins active in G1 or G2 phase. Aim 1b will test the hypothesis that the influence of the cycle on cerebellar neural differentiation may be mediated through G1 active proteins via their control of the E2F transcription factor family. Aim 2 will use yeast two hybrid selection to test the hypothesis that D2 cyclin may interact directly with developmentally important gene products to influence cerebellar pattern formation. Aim 2 will use mice that lack D2 cyclin, D2 cyclin or both to examine their role in cerebellar development, taking advantage of in vivo and in vitro experimental systems. Together, the proposed experiments will provide insights into mechanisms that integrate the control of proliferation and differentiation in cerebellar neural lineages.